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Paris Metro line 3 towards Pont de Levallois-Becon till Opera station, then Metro 8 direction Balard until Ecole-Militaire, which puts you close the Champ de Mars, the large rectangular park on which the Eiffel Tower sits on the far end.

For an RER C train station, say Saint Michel Notre Dame, it would be Metro 3 dir. Pont de Levallois-Becon, transfer at Reamur-Sebastapol to Metro 4 dir. Marie de Montrouge until Saint Michel Notre Dame, where you can transfer to the RER station.

You can also take the RER C from Saint Michel Notre Dame towards Versailles Rive Gauche and get off at Champ de Mars-Tour Eiffel which is close to the Eiffel Tower as well.